I was looking for an answer to this too but couldn't find one! So I made this code from the calendly one, and am posting it here in case it helps someone else. Obviously change the style to suit your needs. If anyone has a better way to do this let me know. This is my first squarespace site so I may have missed an easy trick!
<!-- Calendly link widget begin -->
<center>
<link href="https://assets.calendly.com/assets/external/widget.css" rel="stylesheet">
<script src="https://assets.calendly.com/assets/external/widget.js" type="text/javascript"></script>
<a href="" onclick="Calendly.initPopupWidget({url: 'YOUR CALENDLY LINK GOES HERE'});return false;"
style="padding: 21px 34px;
font-weight: 600;
letter-spacing: 1px;
font-family: sans-serif;
font-size: 15px;
font-weight: 500;
font-style: normal;
text-transform: uppercase;
letter-spacing: .15em;
color: #fff;
background-color: #89c8cd;
border-color: #89c8cd;
text-decoration:none;
border-radius: 3px;"
>YOUR TEXT HERE</a>
</center>
<!-- Calendly link widget end -->